1. Field of the Invention
The present invention relates to a printer for printing information, such as bar codes, on a paper sheet transported between a platen and a print head, by means of the print head.
2. Description of the Related Art
The assignee of the present invention proposes printers in U.S. patent applications U.S. Ser. No. 595,355 filed Oct. 9, 1990 now U.S. Pat. No. 5,085,533 and U.S. Ser. No. 596,093 filed Oct. 11, 1990. These printers comprise a printing section which includes a fixed print head and a platen movable in directions to touch and leave the head. The platen is in the form of a roller, whose opposite end portions are supported individually by means of a pair of arm-shaped platen frames. These frames are rockably mounted on a pivotal shaft, which is supported on a body frame of the printer. The platen roller is urged to be always pressed against the print head, by means of a torsion spring wound on the pivotal shaft, through the medium of the platen frames.
The spring force of the torsion spring is kept constant, so that the force (printing pressure) of the platen roller to press a paper sheet against the print head is also constant.
There are various printing papers which are different in thickness or hardness. Printing on thick or hard paper sheets requires a higher printing pressure than that on thin or soft paper sheets. If the spring force of the torsion spring is fixed, therefore, the printing pressure is liable to be insufficient for thick or hard paper printing. Thus, it is impossible to secure uniform print quality without regard to the thickness or hardness of the printing paper.
In the printer described above, moreover, the platen roller is supported by means of the platen frames which are rockable around the pivotal shaft. In the thick paper printing, the platen frame is rocked in the direction to recede from the print head for a distance corresponding to the paper thickness. Accordingly, the platen roller shifts its position, with respect to the print head, in the feeding direction of the paper sheet which passes between the head and the roller. In other words, the position of the platen roller relative to the print head changes in the paper feeding direction, depending on the paper thickness. Further, in the hard paper printing, since the passage of the paper is curved at the position of the platen roller, the paper applies to the platen roller with force urging the platen roller downward. Thus, as in the case of the thick paper printing, the platen roller is rocked in the direction to leave the print head and shifts its position, with respect to the print head, in the feeding direction of the paper sheet.
Consequently, the proper positional relationship between the print head and the platen is affected, so that the print quality is likely to lower. Thus, in the conventional arrangement, available papers for printing are limited.